[goatlover]

Use ids when JS needs to reference unique elements. Use classes for styling and accessing groups.

[isleyaardvark]

JS can do just as well with unique classnames, which avoids issues with ids like those given in the article.

[sgc]

I always presumed this would usually entail a performance hit, since you are accessing something that is not defined as unique.

[isleyaardvark]

It's an infinitesimal performance decrease, and it avoids a lot of other issues.